Because we must consider dynamic water balance in process of hydrological cycle which takes a long time, the change of basin storage is estimated in caldera basin of volcano Aso.
Then, in water balance equation (1)
R(t), D(t) and ds represent basin reeharge, basin discharge and change of basin storage, dt is a period.
Equation (1) is transformed following equation,
P, ET, Q,4s and G represent Precipitation, Evapotranspiration, depth of discharge through the channel, change of basin storage, and depth of difference between unobserved inflow into basin and outflow of basin.
In equation (2), ususlly we can estimate values of left side terms by observation. Then it is found out that in process of calculation of water balance, we can estimate unknown G in the application of trend in time series, on the other hand that Σ Δs (change of basin storage) correspond to change of piezomteric surface of ground water.